The present invention relates to an organic electroluminescent device and, in particular, a color electroluminescent device.
Electroluminescent (xe2x80x9cELxe2x80x9d) devices are becoming increasingly popular as visual devices, due in-part to cheaper fabrication and longer life provided by improvements in thin-film technology. Typically, EL devices are formed of a number of transparent layers, including an EL layer which emits light when electricity flows through it. In addition to the EL layer, the devices also generally include a substrate and two electrodes on top of the substrate with the EL layer situated between the electrodes. The EL layer can be formed from either inorganic or organic EL materials, each having its own chemistries, fabrication procedures, advantages and disadvantages. EL devices are typically made to be either static or addressable; a static EL device has a single display element which is turned xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d as a single unit to present a single picture, while an addressable EL device includes separately controlled pixels which can be used to generate nearly any picture or display.
EL devices are also sometimes used for color display, but only with difficulty in the case of organic EL devices. This difficulty stems from the facts that (a) the light generating layers (electrodes and organic EL material) generally require a protective seal against moisture and oxygen before the device can be removed from a vacuum or subjected to color patterning, (b) a protective material suitable for such purpose typically uses high temperature fabrication methods in order to achieve a good density that can resist moisture and oxygen, and (c) high temperature processes generally destroy or damage organic EL materials. It would perhaps be simplest if one could deposit light generating layers (the electrodes and organic EL material) on top of a substrate, and color filters additionally on top of the light generating layers, but since the color filter patterning process often involves moving the device during fabrication, as well as use of photolithography and water based solutions, the aforementioned difficulties have generally prevented fabrication of color filters over the organic EL layers.
Two approaches have therefore generally been used for making color organic EL devices. In a first approach, color filters are first deposited upon a transparent substrate, and then are sealed and planarized. The light generating layers are then deposited over the color filters; in this case, display occurs through the transparent substrate and through the color filters. In a second approach, different organic EL materials are used to each generate specially colored light. This second approach is difficult and expensive because of processes used to deposit and pattern the different organic EL materials.
The difficulties mentioned above only increase in the case of addressable devices, since significant additional, generally more complicated patterning must be performed to separately control each of three colors within each image pixel, making it difficult to obtain high image resolution.
A need exists for an organic EL device which features color patterning on top of light generating layers. Further still, a need exists for an organic EL device which does not require use of a thin, transparent substrate (such as in the case where color filters are positioned below the light generating layers or on an opposite side of the substrate from the light generating layers); ideally, such a device would permit the use of opaque substrates or more conventional substrates. Finally, a need exists for a method of fabricating a transparent protective layer for an organic EL device which can resist moisture and support patterning or etching processes. The present invention solves these needs and provides further, related advantages.
The present invention solves the aforementioned needs by providing an organic electroluminescent (xe2x80x9cELxe2x80x9d) device where color filters can be placed over the light generating layers, and where an opaque substrate can be used. With the present invention, a color organic EL device can be fabricated with less complication and expense, e.g., without having to use a thin, transparent substrate, and without having to use different EL materials in order to produce different colors. As should be apparent, therefore, the present invention promotes easier, less costly EL device fabrication.
One form of the present invention provides an organic EL device having a substrate, light generating layers including two electrodes and an organic EL material, and a protective layer above the light generating layers. The protective layer serves as a base above which color filters can be mounted or fabricated and is preferably created using a cold semiconductor fabrication process. xe2x80x9cLow temperaturexe2x80x9d or xe2x80x9ccoldxe2x80x9d as used herein means that the substrate beneath the protective layer is never heated to or above a temperature which causes significant degradation of the organic EL material; in the case of the preferred EL materials, this temperature is about one hundred and forty degrees Celsius.
Because use of some cold processes could potentially lead to defects or pinholes in the protective layer which are not moisture resistant, more detailed aspects of the invention call for fabrication of the protective layer using a conformal deposition process, preferably a plasma deposition process having a sufficiently high ratio of ions to reactive species.
Second and third forms of the invention respectively define a method and an improvement that correspond roughly to the principles outlined above.
The invention may be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings. The detailed description of a particular preferred embodiment, set out below to enable one to build and use one particular implementation of the invention, is not intended to limit the enumerated claims, but to serve as a particular example thereof.